There have been many recent developments in active noise cancellation to improve the generation of cancellation signals emitted into a conduit at a location where the propagating noise wave is 180.degree. out of phase with the introduced sound cancellation signal. While some previously known improvements to the signal control circuitry and are discussed in previously known patent references, these sound cancellation systems do not address protection of the transducer from a destructive environment such as motor vehicle exhaust conduits. Rather previously known improvements to the control 60, for example, enabling it to react to changing characteristics of the sound pressure pulses due to changes at the source, or other improvements such as improved positioning or alignment of components to avoid feedback of the signal generated from the loudspeaker which is received at the transducer 12, or error compensation devices which readjust the control 60 in response to the actual degree of cancellation resulting from operation of a transducer, show that previous developments exhibit a substantially different emphasis for development of noise cancellation systems.
My previous patent applications cover transducer arrangements in which transducers are mounted in housings outside of the exhaust conduit but communicating with the conduit through elongated ports. Although the limited fluid communication through the port and the physical separation of the housing from the conduit provides some reduction in temperatures to which the transducer is subjected, the transducer remains exposed to gases or fluids passing through the conduit. In particular embodiments, such as motor vehicle exhaust systems, such exposure substantially reduces the life of the transducer.
For example, the sleeve carrying the transducer coil is joined to the transducer diaphragm by bonding, glue or other securing means which can be adversely affected by high temperature, humidity or contamination. Moreover, the joint is subjected to forces, stress reversals, aging and cycling during operation of the transducer. Accordingly, the joint may be recognized as a key part of the transducer to protect from environmental conditions affecting the integrity of the joint.